The present invention relates to an image signal decoder for decoding a coded image signal and converting to the previous image signal of the coding, and an image signal display system for decoding a coded image signal, converting to the previous image signal of the coding, and displaying the image signal on a display device, and a liquid crystal display (hereinafter, referred to as LCD) is used as the display device.
Examples of the known apparatuses which highly efficiently code or decode image signals are based on the standards such as the ISO/IEC 13818-2 xe2x80x9cCoding of Audio, Picture, Multimedia and Hypermedia Informationxe2x80x9d and ISO/IEC DIS 11172 xe2x80x9cCoding of Moving Pictures and Associated Audio ISO/IEC JTC1/SC29 WG11xe2x80x9d.
A conventional apparatus for decoding the image signal (hereinafter is referred to as image signal decoder) will be described with reference to FIG. 6. FIG. 6 is a block diagram showing the example of the conventional apparatus for decoding the image signal. In FIG. 6, the output terminal of a variable length decoding section 601 to which a coded image signal is input is connected to the input terminal of an inverse quantization section 602. The output terminal of the inverse quantization section 602 is connected to the input terminal of an inverse discrete cosine transform (hereinafter, referred to as DCT) section 603. The output terminal of the inverse DCT section 603 is connected to the input terminal of a motion compensation section 604. The output terminal of the motion compensation section 604 is connected to the input terminal of a frame buffer 605. The output terminal of the frame buffer 605 is connected to the input terminal of the motion compensation section 604. A decoded image signal is output from the output terminal of the motion compensation section 604.
Decoding operation of the conventional image signal decoder configured above will be described. The input image signal is image data highly efficiently coded based on the ISO/IEC 13818-2 (hereinafter, referred to as coded data). On the input coded data, variable length decoding is performed by the variable length decoding section 601. By the variable length decoding, the following pieces of information are extracted: motion vector information for motion compensation; coded image signal coefficient information; time information for playback stored in the header; and header information representative of the coding mode of each frame and the like.
The coefficient information includes quantized coefficient data and quantization scale data used for the quantization. On the coefficient information, inverse quantization is performed by the inverse quantization section 602, so that the coefficient information is restored to the original DCT coefficient information which was converted into the coefficient information through quantization. On the DCT coefficient information, inverse DCT is performed by the inverse DCT section 603, so that the DCT coefficient information is converted into the original pixel value information which was converted into the DCT coefficient information through orthogonal transformation.
When the pixel value information is that of a frame on which intra-frame coding is performed (hereinafter, referred to as I frame), the pixel value information is output without undergoing the motion compensation by the motion compensation section 604. When the pixel value information is that of a frame on which forward predictive coding is performed (hereinafter, referred to as P frame) or of a frame on which bidirectional predictive coding is performed (hereinafter, referred to B frame), the pixel value information undergoes the motion compensation. That is, the converted pixel value information undergoes the motion compensation by the motion compensation section 604 by use of the motion vector information extracted by the variable length decoding section 601, and are successively output in accordance with the time information for playback. Determining the coding mode of the frame of the pixel value information, which has been, converted by the inverse DCT section 603 (whether the frame is the I frame, the P frame or the B frame) is made based on the header information.
When the pixel value information output from the motion compensation section 604 is that of the I frame or the P frame, it is temporarily stored in the frame buffer 605 so that it is used for the next motion compensation. The frame buffer 605 is capable of storing a maximum of two frames of reference data so that bidirectionally predictive-coded data can be decoded. The frame buffer 605 has a ring buffer configuration in which when the newest frame data is input, it is stored by overwriting the frame data being oldest in time with it.
In this manner, an image signal coded by a hybrid coding method that uses both intra-frame coding and interframe coding is decoded into pixel value information and is output to the display device.
A conventional image signal display system for displaying a decoded image signal on a display device will be described with reference to FIG. 7 and FIG. 8. This image signal display system is disclosed, for example, in Japanese Laid-open Patent Application No. Hei 10-11021.
FIG. 7 is a block diagram showing the configuration of the conventional image signal display system. In FIG. 7, the output terminal of a variable length decoding section 701 to which coded data is input is connected to the input terminal of an inverse quantization section 702. The output terminal of the inverse quantization section 702 is connected to the input terminal of an inverse DCT section 703. The output terminal of the inverse DCT section 703 is connected to the input terminal of a motion compensation section 704. The output terminal of the motion compensation section 704 is connected to a frame buffer 705, an image analyzation section 706 and an output signal correction section 707. The output terminal of the frame buffer 705 is connected to the input terminal of the motion compensation section 704. The output terminal of the image analyzation section 706 is connected to the input terminal of the output signal correction section 707. An output image signal of the output signal correction section 707 is input to an image display section 708.
FIG. 8 is a block diagram showing the configuration of the output signal correction section 707. In FIG. 8, the image signal and signal level distribution information described later are input to a level correction section 801. The output terminal of the level correction section 801 is connected to the input terminal of an RGB conversion section 802. The output terminal of the RGB conversion section 802 is connected to a gamma correction section 803.
Next, the operation of the conventional image signal display system will be described with reference to FIG. 7. On the input coded data, variable length decoding is performed by the variable length decoding section 701. By the variable length decoding, the following pieces of information are extracted: motion vector information for motion compensation; coded image signal coefficient information; time information for playback stored in the header; and header information representative of the coding mode of each frame and the like.
The coefficient information extracted by the variable length decoding section 701 includes quantized coefficient data and quantization scale data used for the quantization. On the coefficient information, inverse quantization is performed by the inverse quantization section 702, so that the coefficient information is restored to the original DCT coefficient information which was converted into the coefficient information through quantization.
On the DCT coefficient information restored by the inverse quantization section 702, inverse DCT is performed by the inverse DCT section 703, so that the DCT coefficient information is restored to the original pixel value information which was converted into the DCT coefficient information through orthogonal transformation. When the pixel value information converted by the inverse DCT section 703 is that of the I frame, the pixel value information is output without undergoing the motion compensation by the motion compensation section 704. When the pixel value information is that of the P frame or the B frame, the pixel value information undergoes the motion compensation by the motion compensation section 704 by use of the motion vector information extracted by the variable length decoding section 701. Then, the pixel value information is successively output in accordance with the time information extracted by the variable length decoding section 701.
The coding mode of the frame of the pixel value information converted by the inverse DCT section 703 is determined based on the header information extracted by the variable length decoding section 701.
When the pixel value information output from the motion compensation section 704 is that of the I frame or the P frame, it is temporarily stored in the frame buffer 705 so that it is used for the next motion compensation. The frame buffer 705 is capable of storing a maximum of two frames of reference data so that bidirectionally predictive-coded data can be decoded. The frame buffer 705 has a ring buffer configuration in which when the newest frame data is input, it is stored by overwriting the frame data being oldest in time with it.
The image analyzation section 706 analyzes the pixel value information output from the motion compensation section 704, and generates intra-frame signal level distribution information (e.g. information such as the maximum signal level, the minimum signal level and an average signal level). Based on the signal level distribution information output from the image analyzation section 706, the output signal correction section 707 performs output correction on the pixel value information output from the motion compensation section 704. For example, the pixel value information output from the motion compensation section 704 is input to the level correction section 801 of the output signal correction section 707. Concurrently, to the level correction section 801, the signal level distribution information is input from the image analyzation section 706. Based on the signal level distribution information, the level correction section 801 corrects (contrast correction or level correction) the pixel value information so that the maximum and the minimum levels of the pixel value information are the same as the maximum and the minimum output levels that can be displayed by the image display device, respectively.
The pixel value information (image signal) output from the level correction section 801 is converted into RGB signals by the RGB conversion section 802. On the RGB signals, input and output correction (gamma correction) responsive to characteristics of the image display device is performed by the gamma correction section 803. In this manner, the coded data is decoded into an image signal conforming to the characteristics of the display device, is output to the image display section 708 and is displayed, for example, on an LCD monitor.
Examples of conventional image signal decoders for decoding coded image signals include playback-only apparatuses such as video CD players and DVD players. Moreover, dedicated decoder boards intended for playback on personal computers, and decoder software that realizes playback processing in the form of software are known. The image signal display system uses, as the display device, a TV monitor using a TV picture tube, a monitor using a display cathode ray tube (hereinafter, referred to as CRT), an LCD monitor, or a monitor using a plasma display panel (hereinafter, referred to as PDP).
In recent years, for saving space, an image signal display system using a flat-panel LCD monitor or PDP monitor has been required. In an image signal display system using an LCD monitor as the image display device, image quality is degraded due to characteristics inherent in LCD monitors. Since LCD monitors are low in response speed, afterimages are apt to be formed when a vigorously moving picture is displayed. Moreover, since LCD monitors provide display by the dot-matrix method, interlace interference occurs when the image signal of the interlace method is displayed. In addition, since LCD monitors are low in screen illuminance, the displayed image is low in contrast and dark.
An object of the present invention is to provide an image signal display system using an LCD monitor, solving the above-mentioned problems inherent in LCD monitors and being capable of providing display of an image quality equal to that provided by TV monitors and CRT monitors, and to provide an image signal decoder for use in the image signal display system.
An image signal decoder according to the present invention is provided with a variable length decoding section for performing variable length decoding on an input image signal coded by a hybrid coding method that uses both intra-frame coding and interframe coding, and generating motion vector information, coefficient information, time information and header information. The image signal decoder is further provided with an interframe motion determination section for storing the motion vector information output from the variable length decoding section, and determining a magnitude of an interframe motion based on a distribution of the motion vector information. When determining that the interframe motion is large, the interframe motion determination section performs control so that the decoding by the variable length decoding section is suspended. The image signal decoder is further provided with: an inverse quantization section for performing inverse quantization on the coefficient information output from the variable length decoding section; and an inverse DCT section for performing inverse DCT on the inversely quantized coefficient information output from the inverse quantization section. The image signal decoder is further provided with: a motion compensation section for performing motion compensation based on the inversely discrete-cosine-transformed coefficient information output from the inverse DCT section and the motion vector information, and generating an output image signal; and a frame buffer for temporarily storing the output image signal output from the motion compensation section.
According to this image signal decoder, the interframe motion determination section detects the interframe motion based on the distribution condition of the motion vector information. When the interframe motion determination section determines that the interframe motion is large, the frame rate is decreased by suspending the processing by the variable length decoding section. Consequently, the formation of afterimages can be suppressed when a played back image is displayed on an LCD monitor.
An image signal decoder according to another aspect of the present invention is provided with a variable length decoding section for performing variable length decoding on an input image signal coded by a hybrid coding method that uses both intra-frame coding and interframe coding, and generating motion vector information, coefficient information, time information and header information. The image signal decoder is further provided with a decode control section for determining a magnitude of an interframe motion based on the motion vector information and the coefficient information output from the variable length decoding section and the number of bits per frame. When determining that the interframe motion is large, the decode control section suspends the decoding by the variable length decoding section. The image signal decoder is further provided with: an inverse quantization section for performing inverse quantization on the coefficient information output from the variable length decoding section; an inverse DCT section for performing inverse DCT on the inversely quantized coefficient information output from the inverse quantization section; a motion compensation section for performing motion compensation based on the inversely discrete-cosine-transformed coefficient information output from the inverse DCT section and the motion vector information, and generating an output image signal; and a frame buffer for temporarily storing the output image signal output from the motion compensation section.
According to this image signal decoder, the decode control section determines the interframe motion based on the distribution condition of the motion vector information, the number of generated bits per frame and quantization information included in the coefficient information. When the decode control section determines that the interframe motion is large, the frame rate is decreased by suspending the processing in the variable length decoding section. Consequently, the formation of afterimages can be suppressed when a played back image is displayed on an LCD monitor.
An image signal decoder according to still another aspect of the present invention uses both intra-frame coding and interframe coding. The image signal decoder is provided with a variable length decoding section for performing variable length decoding on an input image signal coded by a hybrid coding method that selectively uses frame processing or field processing for each coding unit, and generating motion vector information, coefficient information, time information and header information. The image signal decoder is further provided with a frame/field processing detection-section for determining whether the currently performed coding is performed in units of frames or in units of fields based on the motion vector information and the coefficient information output from the variable length decoding section. The image signal decoder is further provided with; an inverse quantization section for performing inverse quantization on the coefficient information output from the variable length decoding section; and an inverse DCT section for performing inverse DCT on the inversely quantized coefficient information output from the inverse quantization section. The image signal decoder is further provided with: a motion compensation section for performing motion compensation based on the inversely discrete-cosine-transformed coefficient information output from the inverse DCT section and the motion vector information, and generating a first output image signal; a frame buffer for temporarily storing the first output image signal output from the motion compensation section; and a field interpolation section for, when the frame/field processing detection-section determines that the coding is performed in units of fields, interpolating one of field data of the first output image signal output from the motion compensation section, and generating a second output image signal. The image signal decoder is further provided with an output switching section for performing control so that the first output image signal from the motion compensation section is output when the frame/field processing detection-section determines that the coding is performed in units of frames, and that the second output image signal from the field interpolation section is output when the frame/field processing detection-section determines that the coding is performed in units of fields.
According to this image signal decoder, the frame/field processing detection-section determines whether the coding is performed in units of frames or in units of fields for each coding unit. When it is performed in units of fields, the frame/field processing detection-section determines that there is an intra-field motion in the area, and the field interpolation section produces a second output image signal by interpolation and outputs it. Consequently, the occurrence of interlace interference can be suppressed when a played back image is displayed on an LCD monitor.
An image signal display system according to the present invention is provided with a variable length decoding section for performing variable length decoding on an input image signal coded by a hybrid coding method that uses both intra-frame coding and interframe coding, and generating motion vector information, coefficient information, time information and header information. The image signal display system is further provided with: an inverse quantization section for performing inverse quantization on the coefficient information output from the variable length decoding section; and a DC level distribution information detection section for temporarily storing only DC component information of the inversely quantized coefficient information output from the inverse quantization section, generating distribution information of the DC component information corresponding to one frame, and outputting the generated distribution information. The image signal display system is further provided with: an inverse DCT section for performing inverse DCT on the inversely quantized coefficient information output from the inverse quantization section; and a motion compensation section for performing motion compensation based on the inversely discrete-cosine-transformed coefficient information output from the inverse DCT section and the motion vector information, and generating a first output image signal. The image signal display system is further provided with: a frame buffer for temporarily storing the first output image signal output from the motion compensation section; an output signal correction section for correcting the first output image signal output from the motion compensation section based on the distribution information of the DC component information output from the DC level distribution information detection section, and generating a second output image signal; and an image display section for displaying the second output image signal output from the output signal correction section.
According to the image signal display system, the DC level distribution information detection section produces the distribution information of the DC component corresponding to one frame in decoding, and the distribution information is used for the output signal correction of the decoded image signal. Consequently, image analysis conventionally performed after decoding is unnecessary, so that the overall processing amount can be reduced and the apparatus scale can be reduced. In addition, even in the case where an LCD monitor is used as the display device, high quality display is realizable so as to be similar to the use of a TV monitor or a CRT monitor as the display device.